Index: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -223,13 +223,14 @@
   }
 
   SmallVector<Register> GCDRegs;
-  LLT GCDTy;
-  for (Register PartReg : PartRegs)
-    GCDTy = extractGCDType(GCDRegs, ResultTy, LeftoverTy, PartReg);
-
-  for (Register PartReg : LeftoverRegs)
-    extractGCDType(GCDRegs, ResultTy, LeftoverTy, PartReg);
 
+  // Populate the first entry in GCDRegs separately in order to avoid
+  // recalculating GCDTy.
+  LLT GCDTy = extractGCDType(GCDRegs, ResultTy, LeftoverTy, PartRegs[0]);
+  for (auto PartReg : make_range(std::next(PartRegs.begin()), PartRegs.end()))
+    extractGCDType(GCDRegs, GCDTy, PartReg);
+  for (auto PartReg : LeftoverRegs)
+    extractGCDType(GCDRegs, GCDTy, PartReg);
   LLT ResultLCMTy = buildLCMMergePieces(ResultTy, LeftoverTy, GCDTy, GCDRegs);
   buildWidenedRemergeToDst(DstReg, ResultLCMTy, GCDRegs);
 }
Index: llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
===================================================================
--- llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
+++ llvm/test/CodeGen/AArch64/GlobalISel/legalize-constant.mir
@@ -1,31 +1,11 @@
 # NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
-# RUN: llc -O0 -run-pass=legalizer %s -o - | FileCheck %s
+# RUN: llc -O0 -mtriple aarch64 -run-pass=legalizer -verify-machineinstrs %s -o - | FileCheck %s
 
---- |
-  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
-  target triple = "aarch64--"
-  define void @test_constant() {
-  entry:
-    ret void
-  }
-  define void @test_fconstant() {
-  entry:
-    ret void
-  }
-...
-
----
 name:            test_constant
 registers:
-  - { id: 0, class: _ }
-  - { id: 1, class: _ }
-  - { id: 2, class: _ }
-  - { id: 3, class: _ }
-  - { id: 4, class: _ }
-  - { id: 5, class: _ }
 body: |
-  bb.0.entry:
-
+  bb.0:
+    liveins: $x0
     ; CHECK-LABEL: name: test_constant
     ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
     ; CHECK: $w0 = COPY [[C]](s32)
@@ -39,32 +19,28 @@
     ; CHECK: $x0 = COPY [[C3]](s64)
     ; CHECK: [[C4:%[0-9]+]]:_(s64) = G_CONSTANT i64 0
     ; CHECK: $x0 = COPY [[C4]](s64)
-    %0(s1) = G_CONSTANT i1 0
+    %0:_(s1) = G_CONSTANT i1 0
     %6:_(s32) = G_ANYEXT %0
     $w0 = COPY %6
-    %1(s8) = G_CONSTANT i8 42
+    %1:_(s8) = G_CONSTANT i8 42
     %7:_(s32) = G_ANYEXT %1
     $w0 = COPY %7
-    %2(s16) = G_CONSTANT i16 65535
+    %2:_(s16) = G_CONSTANT i16 65535
     %8:_(s32) = G_ANYEXT %2
     $w0 = COPY %8
-    %3(s32) = G_CONSTANT i32 -1
+    %3:_(s32) = G_CONSTANT i32 -1
     $w0 = COPY %3
-    %4(s64) = G_CONSTANT i64 1
+    %4:_(s64) = G_CONSTANT i64 1
     $x0 = COPY %4
-    %5(s64) = G_CONSTANT i64 0
+    %5:_(s64) = G_CONSTANT i64 0
     $x0 = COPY %5
 ...
 
 ---
 name:            test_fconstant
-registers:
-  - { id: 0, class: _ }
-  - { id: 1, class: _ }
-  - { id: 2, class: _ }
 body: |
-  bb.0.entry:
-
+  bb.0:
+    liveins: $x0
     ; CHECK-LABEL: name: test_fconstant
     ; CHECK: [[C:%[0-9]+]]:_(s32) = G_FCONSTANT float 1.000000e+00
     ; CHECK: $w0 = COPY [[C]](s32)
@@ -74,11 +50,76 @@
     ; CHECK: [[FPTRUNC:%[0-9]+]]:_(s16) = G_FPTRUNC [[C2]](s32)
     ; CHECK: [[ANYEXT:%[0-9]+]]:_(s32) = G_ANYEXT [[FPTRUNC]](s16)
     ; CHECK: $w0 = COPY [[ANYEXT]](s32)
-    %0(s32) = G_FCONSTANT float 1.0
+    %0:_(s32) = G_FCONSTANT float 1.0
     $w0 = COPY %0
-    %1(s64) = G_FCONSTANT double 2.0
+    %1:_(s64) = G_FCONSTANT double 2.0
     $x0 = COPY %1
-    %2(s16) = G_FCONSTANT half 0.0
+    %2:_(s16) = G_FCONSTANT half 0.0
     %3:_(s32) = G_ANYEXT %2
     $w0 = COPY %3
 ...
+
+---
+name:           s792
+registers:
+body: |
+  bb.0:
+    liveins: $x0
+    ; CHECK-LABEL: name: s792
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 0
+    ; CHECK: [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; CHECK: [[UV:%[0-9]+]]:_(s8), [[UV1:%[0-9]+]]:_(s8), [[UV2:%[0-9]+]]:_(s8), [[UV3:%[0-9]+]]:_(s8), [[UV4:%[0-9]+]]:_(s8), [[UV5:%[0-9]+]]:_(s8), [[UV6:%[0-9]+]]:_(s8), [[UV7:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV8:%[0-9]+]]:_(s8), [[UV9:%[0-9]+]]:_(s8), [[UV10:%[0-9]+]]:_(s8), [[UV11:%[0-9]+]]:_(s8), [[UV12:%[0-9]+]]:_(s8), [[UV13:%[0-9]+]]:_(s8), [[UV14:%[0-9]+]]:_(s8), [[UV15:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV16:%[0-9]+]]:_(s8), [[UV17:%[0-9]+]]:_(s8), [[UV18:%[0-9]+]]:_(s8), [[UV19:%[0-9]+]]:_(s8), [[UV20:%[0-9]+]]:_(s8), [[UV21:%[0-9]+]]:_(s8), [[UV22:%[0-9]+]]:_(s8), [[UV23:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV24:%[0-9]+]]:_(s8), [[UV25:%[0-9]+]]:_(s8), [[UV26:%[0-9]+]]:_(s8), [[UV27:%[0-9]+]]:_(s8), [[UV28:%[0-9]+]]:_(s8), [[UV29:%[0-9]+]]:_(s8), [[UV30:%[0-9]+]]:_(s8), [[UV31:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV32:%[0-9]+]]:_(s8), [[UV33:%[0-9]+]]:_(s8), [[UV34:%[0-9]+]]:_(s8), [[UV35:%[0-9]+]]:_(s8), [[UV36:%[0-9]+]]:_(s8), [[UV37:%[0-9]+]]:_(s8), [[UV38:%[0-9]+]]:_(s8), [[UV39:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV40:%[0-9]+]]:_(s8), [[UV41:%[0-9]+]]:_(s8), [[UV42:%[0-9]+]]:_(s8), [[UV43:%[0-9]+]]:_(s8), [[UV44:%[0-9]+]]:_(s8), [[UV45:%[0-9]+]]:_(s8), [[UV46:%[0-9]+]]:_(s8), [[UV47:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV48:%[0-9]+]]:_(s8), [[UV49:%[0-9]+]]:_(s8), [[UV50:%[0-9]+]]:_(s8), [[UV51:%[0-9]+]]:_(s8), [[UV52:%[0-9]+]]:_(s8), [[UV53:%[0-9]+]]:_(s8), [[UV54:%[0-9]+]]:_(s8), [[UV55:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV56:%[0-9]+]]:_(s8), [[UV57:%[0-9]+]]:_(s8), [[UV58:%[0-9]+]]:_(s8), [[UV59:%[0-9]+]]:_(s8), [[UV60:%[0-9]+]]:_(s8), [[UV61:%[0-9]+]]:_(s8), [[UV62:%[0-9]+]]:_(s8), [[UV63:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV64:%[0-9]+]]:_(s8), [[UV65:%[0-9]+]]:_(s8), [[UV66:%[0-9]+]]:_(s8), [[UV67:%[0-9]+]]:_(s8), [[UV68:%[0-9]+]]:_(s8), [[UV69:%[0-9]+]]:_(s8), [[UV70:%[0-9]+]]:_(s8), [[UV71:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV72:%[0-9]+]]:_(s8), [[UV73:%[0-9]+]]:_(s8), [[UV74:%[0-9]+]]:_(s8), [[UV75:%[0-9]+]]:_(s8), [[UV76:%[0-9]+]]:_(s8), [[UV77:%[0-9]+]]:_(s8), [[UV78:%[0-9]+]]:_(s8), [[UV79:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV80:%[0-9]+]]:_(s8), [[UV81:%[0-9]+]]:_(s8), [[UV82:%[0-9]+]]:_(s8), [[UV83:%[0-9]+]]:_(s8), [[UV84:%[0-9]+]]:_(s8), [[UV85:%[0-9]+]]:_(s8), [[UV86:%[0-9]+]]:_(s8), [[UV87:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV88:%[0-9]+]]:_(s8), [[UV89:%[0-9]+]]:_(s8), [[UV90:%[0-9]+]]:_(s8), [[UV91:%[0-9]+]]:_(s8), [[UV92:%[0-9]+]]:_(s8), [[UV93:%[0-9]+]]:_(s8), [[UV94:%[0-9]+]]:_(s8), [[UV95:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C]](s64)
+    ; CHECK: [[UV96:%[0-9]+]]:_(s8), [[UV97:%[0-9]+]]:_(s8), [[UV98:%[0-9]+]]:_(s8), [[UV99:%[0-9]+]]:_(s8) = G_UNMERGE_VALUES [[C1]](s32)
+    ; CHECK: [[MV:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV]](s8), [[UV1]](s8), [[UV2]](s8)
+    ; CHECK: [[MV1:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV3]](s8), [[UV4]](s8), [[UV5]](s8)
+    ; CHECK: [[MV2:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV6]](s8), [[UV7]](s8), [[UV8]](s8)
+    ; CHECK: [[MV3:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV9]](s8), [[UV10]](s8), [[UV11]](s8)
+    ; CHECK: [[MV4:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV12]](s8), [[UV13]](s8), [[UV14]](s8)
+    ; CHECK: [[MV5:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV15]](s8), [[UV16]](s8), [[UV17]](s8)
+    ; CHECK: [[MV6:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV18]](s8), [[UV19]](s8), [[UV20]](s8)
+    ; CHECK: [[MV7:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV21]](s8), [[UV22]](s8), [[UV23]](s8)
+    ; CHECK: [[MV8:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV24]](s8), [[UV25]](s8), [[UV26]](s8)
+    ; CHECK: [[MV9:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV27]](s8), [[UV28]](s8), [[UV29]](s8)
+    ; CHECK: [[MV10:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV30]](s8), [[UV31]](s8), [[UV32]](s8)
+    ; CHECK: [[MV11:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV33]](s8), [[UV34]](s8), [[UV35]](s8)
+    ; CHECK: [[MV12:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV36]](s8), [[UV37]](s8), [[UV38]](s8)
+    ; CHECK: [[MV13:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV39]](s8), [[UV40]](s8), [[UV41]](s8)
+    ; CHECK: [[MV14:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV42]](s8), [[UV43]](s8), [[UV44]](s8)
+    ; CHECK: [[MV15:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV45]](s8), [[UV46]](s8), [[UV47]](s8)
+    ; CHECK: [[MV16:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV48]](s8), [[UV49]](s8), [[UV50]](s8)
+    ; CHECK: [[MV17:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV51]](s8), [[UV52]](s8), [[UV53]](s8)
+    ; CHECK: [[MV18:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV54]](s8), [[UV55]](s8), [[UV56]](s8)
+    ; CHECK: [[MV19:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV57]](s8), [[UV58]](s8), [[UV59]](s8)
+    ; CHECK: [[MV20:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV60]](s8), [[UV61]](s8), [[UV62]](s8)
+    ; CHECK: [[MV21:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV63]](s8), [[UV64]](s8), [[UV65]](s8)
+    ; CHECK: [[MV22:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV66]](s8), [[UV67]](s8), [[UV68]](s8)
+    ; CHECK: [[MV23:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV69]](s8), [[UV70]](s8), [[UV71]](s8)
+    ; CHECK: [[MV24:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV72]](s8), [[UV73]](s8), [[UV74]](s8)
+    ; CHECK: [[MV25:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV75]](s8), [[UV76]](s8), [[UV77]](s8)
+    ; CHECK: [[MV26:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV78]](s8), [[UV79]](s8), [[UV80]](s8)
+    ; CHECK: [[MV27:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV81]](s8), [[UV82]](s8), [[UV83]](s8)
+    ; CHECK: [[MV28:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV84]](s8), [[UV85]](s8), [[UV86]](s8)
+    ; CHECK: [[MV29:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV87]](s8), [[UV88]](s8), [[UV89]](s8)
+    ; CHECK: [[MV30:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV90]](s8), [[UV91]](s8), [[UV92]](s8)
+    ; CHECK: [[MV31:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV93]](s8), [[UV94]](s8), [[UV95]](s8)
+    ; CHECK: [[MV32:%[0-9]+]]:_(s24) = G_MERGE_VALUES [[UV96]](s8), [[UV97]](s8), [[UV98]](s8)
+    ; CHECK: %cst:_(s792) = G_MERGE_VALUES [[MV]](s24), [[MV1]](s24), [[MV2]](s24), [[MV3]](s24), [[MV4]](s24), [[MV5]](s24), [[MV6]](s24), [[MV7]](s24), [[MV8]](s24), [[MV9]](s24), [[MV10]](s24), [[MV11]](s24), [[MV12]](s24), [[MV13]](s24), [[MV14]](s24), [[MV15]](s24), [[MV16]](s24), [[MV17]](s24), [[MV18]](s24), [[MV19]](s24), [[MV20]](s24), [[MV21]](s24), [[MV22]](s24), [[MV23]](s24), [[MV24]](s24), [[MV25]](s24), [[MV26]](s24), [[MV27]](s24), [[MV28]](s24), [[MV29]](s24), [[MV30]](s24), [[MV31]](s24), [[MV32]](s24)
+    ; CHECK: %trunc:_(s32) = G_TRUNC %cst(s792)
+    ; CHECK: $w0 = COPY %trunc(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %cst:_(s792) = G_CONSTANT i792 0
+    %trunc:_(s32) = G_TRUNC %cst(s792)
+    $w0 = COPY %trunc(s32)
+    RET_ReallyLR implicit $w0
+...